The invention relates to a portable dosimeter for indicating the intensity of radiation and, more particularly, high-intensity radiation used in medical therapies.
High-energy accelerators for producing high-energy electron, photon, and other particle beams are finding increasingly widespread uses, for example in medical radiation therapies. The intensity of the radiation from these accelerators varies over time for a number of well-known reasons. The intensity therefore must be checked periodically because many of the uses of the radiation require a highly-accurate radiation intensity. Medical radiation therapy, for example, requires that the dose of the radiation on the body target be determined precisely to be effective but not harmful.
In the past, high energy electron, photon, and other particle beams were used only in a few major institutions which had physics staffs capable of investigating the fundamental problems encountered in regulating the intensity of the high energy radiation. Today, however, high energy accelerators frequently are found in small institutions such as community hospitals. These institutions do not have physics staffs adequate for using the Faraday cup, the calorimeter, or the Fricke ionometric dosimeters which provide the basis for absolute dosimetry. In addition, these devices are cumbersome for the everyday calibration of high energy accelerators. Several protocols have therefore been developed for the dosimetry of high energy electron, photon, and other particle beams for routine application in the field. Most of these protocols recommend the use of ionometric methods of dosimetry using commercially available ionization chambers to calibrate the high energy accelerators.
Although the protocols serve to establish consistency in dosimetry methodology, various problems have become apparent in some of the protocols. Some of these problems arise from the design and use of the dosimeters which were developed for implementing the protocols. One problem is chamber and phantoms homogeneity. Another problem is the need to recalibrate known ionometric dosimeters periodically, for example about once a year. Regional calibration laboratories or the United States government National Bureau of Standards are used for recalibrating the dosimeters, but while the dosimeter is being recalibrated, it is not available for calibrating the high energy accelerator. Multiple dosimeters or careful scheduling are therefore required to keep the high energy accelerator available for desired, calibrated use.
One of the inventors named herein and others at the Department of Medical Physics, Memorial Hospital, New York, N.Y., developed a parallel plate or pancake ionization chamber for use in dosimetry which overcomes many of these problems. This pancake ionization chamber is described in detail in the International Journal of Radiation Oncology, Biological Physics, Volume v, pages 2031 to 2038, November-December, 1979.
In general, the pancake ionization chamber has a pair of parallel, graphite electrodes on polystyrene substrates precisely spaced from each other. By spacing the parallel electrodes precisely, the volume of the ionization chamber between the electrodes is fixed and determined precisely. The fixed, precise volume of the ionization chamber, determined both electrically and mechanically, eliminates the need for a periodical radiological recalibrating a dosimeter.
Among the problems which were not solved by the pancake ionization chamber described above were those of shielding the electronic circuitry of the dosimeter instrumentation from the effects of the high intensity radiation. The intensity of the radiation is intended to ionize a gas (air) in the pancake ionization chamber. As described in the publication, the ion chamber may be vented to the atmosphere to eliminate problems of maintaining a sealed cavity. If the radiation ionizes the air in the chamber, however, it also ionizes air adjacent to the chamber, and these ions can affect the electronic circuitry of the dosimeter. The radiation itself can also affect the dosimeter instrumentation, including, for example, many solid state devices which are known to be radiation sensitive.